Inhibitory action of nicotinic antagonists on transmitter release at the neuromuscular junction of the rat

Homeostatic Plasticity of the Mammalian Neuromuscular Junction

The mammalian neuromuscular junction (NMJ) is an ideal preparation to study synaptic plasticity. Its simplicity- one input, one postsynaptic target- allows experimental manipulations and mechanistic analyses that are impossible at more complex synapses. Homeostatic synaptic plasticity attempts to maintain normal function in the face of perturbations in activity. At the NMJ, 3 aspects of activity are sensed to trigger 3 distinct mechanisms that contribute to homeostatic plasticity: Block of presynaptic action potentials triggers increased quantal size secondary to increased release of acetylcholine from vesicles. Simultaneous block of pre- and postsynaptic action potentials triggers an increase in the probability of vesicle release. Block of acetylcholine binding to acetylcholine receptors during spontaneous fusion of single vesicles triggers an increase in the number of releasable vesicles as well as increased motoneuron excitability. Understanding how the NMJ responds to perturbations of synaptic activity informs our understanding of its response to diverse neuromuscular diseases.

Muscle Nicotinic Acetylcholine Receptors May Mediate Trans-Synaptic Signaling at the Mouse Neuromuscular Junction

Block of neurotransmitter receptors at the neuromuscular junction (NMJ) has been shown to trigger upregulation of the number of synaptic vesicles released (quantal content, QC), a response termed homeostatic synaptic plasticity. The mechanism underlying this plasticity is not known. Here, we used selective toxins to demonstrate that block of α1-containing nicotinic acetylcholine receptors (nAChRs) at the NMJ of male and female mice triggers the upregulation of QC. Reduction of current flow through nAChRs, induced by drugs with antagonist activity, demonstrated that reduction in synaptic current per se does not trigger upregulation of QC. These data led to the remarkable conclusion that disruption of synaptic transmission is not sensed to trigger upregulation of QC. During studies of the effect of partial block of nAChRs on QC, we observed a small but reproducible increase in the decay kinetics of miniature synaptic currents. The change in kinetics was correlated with the increase in QC and raises the possibility that a change in postsynaptic nAChR conformation may be associated with the presynaptic increase in QC. We propose that, in addition to functioning in synaptic transmission, ionotropic muscle nicotonic nAChRs may serve as signaling molecules that participate in synaptic plasticity. Because nAChRs have been implicated in a number of disease states, the finding that nAChRs may be involved in triggering synaptic plasticity could have wide-reaching implications.SIGNIFICANCE STATEMENT The signals that initiate synaptic plasticity of the nervous system are still incompletely understood. Using the mouse neuromuscular junction as a model synapse, we studied how block of neurotransmitter receptors is sensed to trigger synaptic plasticity. Our studies led to the surprising conclusion that neither changes in synaptic current nor spiking of the presynaptic or postsynaptic cell are sensed to initiate synaptic plasticity. Instead, postsynaptic nicotinic acetylcholine receptors (nAChRs), in addition to functioning in synaptic transmission, may serve as signaling molecules that trigger synaptic plasticity. Because nAChRs have been implicated in a number of disease states, the finding that they may mediate synaptic plasticity has broad implications.

Homeostatic synaptic plasticity at the neuromuscular junction in myasthenia gravis

A number of studies in the past 20 years have shown that perturbation of activity of the nervous system leads to compensatory changes in synaptic strength that serve to return network activity to its original level. This response has been termed homeostatic synaptic plasticity. Despite the intense interest in homeostatic synaptic plasticity, little attention has been paid to its role in the prototypic synaptic disease, myasthenia gravis. In this review, we discuss mechanisms that have been shown to mediate homeostatic synaptic plasticity at the mammalian neuromuscular junction. A subset of these mechanisms have been shown to occur in myasthenia gravis. The homeostatic changes occurring in myasthenia gravis appear to involve the presynaptic nerve terminal and may even involve changes in the excitability of motor neurons within the spinal cord. The finding of presynaptic homeostatic synaptic plasticity in myasthenia gravis leads us to propose that changes in the motor unit in myasthenia gravis may be more widespread than previously appreciated.

Reversible Recruitment of a Homeostatic Reserve Pool of Synaptic Vesicles Underlies Rapid Homeostatic Plasticity of Quantal Content

Homeostatic regulation is essential for the maintenance of synaptic strength within the physiological range. The current study is the first to demonstrate that both induction and reversal of homeostatic upregulation of synaptic vesicle release can occur within seconds of blocking or unblocking acetylcholine receptors at the mouse neuromuscular junction. Our data suggest that the homeostatic upregulation of release is due to Ca(2+)-dependent increase in the size of the readily releasable pool (RRP). Blocking vesicle refilling prevented upregulation of quantal content (QC), while leaving baseline release relatively unaffected. This suggested that the upregulation of QC was due to mobilization of a distinct pool of vesicles that were rapidly recycled and thus were dependent on continued vesicle refilling. We term this pool the "homeostatic reserve pool." A detailed analysis of the time course of vesicle release triggered by a presynaptic action potential suggests that the homeostatic reserve pool of vesicles is normally released more slowly than other vesicles, but the rate of their release becomes similar to that of the major pool during homeostatic upregulation of QC. Remarkably, instead of finding a generalized increase in the recruitment of vesicles into RRP, we identified a distinct homeostatic reserve pool of vesicles that appear to only participate in synchronized release following homeostatic upregulation of QC. Once this small pool of vesicles is depleted by the block of vesicle refilling, homeostatic upregulation of QC is no longer observed. This is the first identification of the population of vesicles responsible for the blockade-induced upregulation of release previously described. Significance statement: The current study is the first to demonstrate that both the induction and reversal of homeostatic upregulation of synaptic vesicle release can occur within seconds. Our data suggest that homeostatic upregulation of release is due to Ca(2+)-dependent priming/docking of a small homeostatic reserve pool of vesicles that normally have slow-release kinetics. Following priming, the reserve pool of vesicles is released synchronously with the normal readily releasable pool of synaptic vesicles. This is the first description of this unique pool of synaptic vesicles.

Blockade of neuronal facilitatory nicotinic receptors containing alpha 3 beta 2 subunits contribute to tetanic fade in the rat isolated diaphragm

Nicotinic receptor (nAChR) subtypes involved in pre- and postjunctional actions underlying tetanic fade were studied in rat phrenic-nerve hemidiaphragms. We investigated the ability of subtype-specific nAChR antagonists to depress nerve-evoked contractions and [(3)H]-acetylcholine ([(3)H]-ACh) release. Muscle tension was transiently increased during brief high frequency trains (50 Hz for 5 sec). The rank potency order of nAChR antagonists to reduce tetanic peak tension was alpha-bungarotoxin > d-tubocurarine >> mecamylamine > hexamethonium. Reduction of maximal tetanic tension produced by dihydro-beta-erythroidine (0.03-10 microM), methyllycaconitine (0.003-3 microM), and alpha-conotoxin MII (0.001-0.3 microM) did not exceed 30%. Besides reduction of peak tension d-tubocurarine (0.1-0.7 microM), mecamylamine (0.1-300 microM), and hexamethonium (30-3,000 microM) also caused tetanic fading. With alpha-conotoxin MII (0.001-0.3 microM) and dihydro-beta-erythroidine (0.03-10 microM), tetanic fade was evident only after decreasing the safety factor of neuromuscular transmission (with high magnesium ions, 6-7 mM). The antagonist rank potency order to reduce evoked (50 Hz for 5 sec) [(3)H]-ACh release from motor nerve terminals was alpha-conotoxin MII (0.1 microM) > dihydro-beta-erythroidine (1 microM) approximately d-tubocurarine (1 microM) > mecamylamine (100 microM) > hexamethonium (1,000 microM). When applied in a concentration (0.3 microM) above that producing tetanic paralysis, alpha-bungarotoxin failed to affect [(3)H]-ACh release. Data obtained suggest that postjunctional neuromuscular relaxants interact with alpha-bungarotoxin-sensitive nicotinic receptors containing alpha1-subunits, whereas blockade of neuronal alpha3beta2-containing receptors produce tetanic fade by breaking nicotinic autofacilitation of acetylcholine release.

Adenosine inhibits N-type calcium channels at the rat neuromuscular junction

1. In earlier studies, it has been reported that under in vitro conditions transmitter release at the rat neuromuscular junction is normally suppressed due to the effect of adenosine release from the isolated tissue. In the present study we wanted to determine whether this action may involve the inhibition of calcium influx through adenosine-sensitive calcium channels. 2. In order to test this hypothesis, we examined the role of N-type calcium channels in regulating nerve-evoked transmitter release by using the N-type calcium channel-specific blocker omega-conotoxin GVIA (CTX). In order to control the inhibitory action of adenosine, we also used the adenosine A1 receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX). We tested the effect of blocking N-type calcium channels with CTX in the presence and absence of DPCPX. We examined the effects of these drugs on quantal transmitter release in the transected preparation of the phrenic nerve-hemidiaphragm of the rat using intracellular recording techniques. 3. At 10 nmol/L, CTX alone had no effect on nerve-evoked transmitter release; however, in the presence of 0.1 micro mol/L DPCPX, CTX significantly depressed nerve-evoked transmitter release. 4. These data support the view that adenosine inhibits nerve-evoked transmitter release by inhibiting N-type calcium channels on nerve terminals.

Studies on curare-like action of the tripeptide carbobenzoxy-Gly-Gly-Arg-beta-naphthylamide in mouse diaphragm

The effects of several protease substrates or protease inhibitors on neuromuscular transmission in the isolated mouse phrenic nerve-diaphragm were studied. N-Carbobenzoxy-Gly-Gly-Arg-beta-naphthylamide (Z-GGR-N) but none of the other agents inhibited the nerve-evoked muscle contractions. By means of electrophysiological studies, Z-GGR-N was found to inhibit the amplitudes of both end-plate potentials (epps) (IC50 approximately 50 microM) and miniature end-plate potentials (mepps) but to increase the frequencies of mepps. This tripeptide could protect the nicotinic acetylcholine receptor from the irreversible inhibitory action of alpha-bungarotoxin on the mouse diaphragm. Similar to D-tubocurarine, Z-GGR-N induced tetanic fading both of nerve-evoked muscle contractions and of the amplitude of epps. Furthermore, Z-GGR-N exhibited a greater depression of the amplitudes of train-epps than those of mepps, similar to that of hexamethonium and D-tubocurarine, indicating an effect on presynaptic autoreceptors. Suramin, which could competitively reverse the inhibitory effects of non-depolarizing relaxants, acted in this study as an antagonist of all the effects of Z-GGR-N, especially those at the presynaptic site. All of these findings suggest that Z-GGR-N is a novel tripeptide possessing curare-like actions at both presynaptic and postsynaptic sites and that these actions are independent of its protease substrate property.

Simultaneous comparison of nicotinic receptor antagonists on three nicotinic acetylcholine receptors

The relative potencies of several nicotinic cholinoceptor antagonists in producing tetanic fade and reduction of striated muscle contraction were investigated in the isolated guinea-pig oesophagus as well as the guinea-pig and rat phrenic nerve-diaphragm preparations. Contractile smooth muscle responses to vagal stimulation, which involves ganglionic activation, were also measured simultaneously with striated muscle responses in the oesophagus. The relative potency for inhibiting the response of oesophageal smooth muscle to vagal stimulation (20 Hz) was trimetaphan > mecamylamine > hexamethonium > tubocurarine > pancuronium. For oesophageal striated muscle, production of tetanic fade at 100 Hz and reduction in peak tetanic tension at 20 or 100 Hz showed a similar relative potency; pancuronium > tubocurarine > mecamylamine > trimetaphan > hexamethonium and similar results were obtained in the guinea-pig diaphragm for the antagonists investigated (pancuronium, tubocurarine and mecamylamine). In the rat phrenic nerve-diaphragm preparation, production of tetanic fade at 50 Hz and reduction in twitch or tetanic tension all showed the relative potency; tubocurarine > pancuronium > mecamylamine > trimetaphan > hexamethonium. These findings indicate differences in the nicotinic cholinoceptor subtypes involved in vagal ganglionic responses and those in tetanic fade.

Impact of alpha-bungarotoxin on transmitter release at the neuromuscular junction of the rat

The drug, alpha-bungarotoxin (BTX) is believed to be a 'pure' nicotinic antagonist. Hence, use of this drug should avoid the secondary actions associated with other nicotinic antagonists. The hypothesis that the motor nerve terminal responds to the presence of acetylcholine (ACh) by releasing less transmitter was tested by examining the effects of BTX on end-plate potentials (EPPs), miniature end-plate potentials (MEPPs), and quantal release at the rat diaphragm neuromuscular junction. Analysis of EPP and MEPP amplitudes and quantal release demonstrate that BTX significantly increases transmitter release at the onset of tetanic stimulation (50 Hz). Like other nicotinic antagonists, BTX was not able to sustain enhanced quantal release during a brief train of 40 stimuli and resulted in greater decline in EPP amplitude during tetanic stimulation. The data suggests that negative feedback regulation by presynaptic autoreceptors only serves a functional role at the onset of stimulation and that other factors such as transmitter supply or adenosine regulation may serve to dominate transmitter release during maintained tetanic stimulation.